Systems and methods for preventing or reducing contamination enhanced laser induced damage (C-LID) to optical components using gas phase additives

ABSTRACT

Systems and methods for preventing or reducing contamination enhanced laser induced damage (C-LID) to optical components are provided including a housing enclosing an optical component, a container configured to hold a gas phase additive and operatively coupled to the housing; and a delivery system configured to introduce the gas phase additive from the container into the housing and to maintain the gas phase additive at a pre-selected partial pressure within the housing. The gas phase additive may have a greater affinity for the optical component than does a contaminant and may be present in an amount sufficient to inhibit laser induced damage resulting from contact between the contaminant and the optical component. The housing may be configured to maintain a sealed gas environment or vacuum.

FIELD OF THE INVENTION

This application generally relates to preventing or reducingcontamination laser induced damage to optical components.

BACKGROUND OF THE INVENTION

It is well known that laser beams may in some circumstances damageoptical materials and coatings. Referred to as Laser Induced Damage(LID), it is believed that damage to optical materials may stem from thedirect interaction of photons with the material. In particular, it isbelieved that the electric field component of the laser radiation mayinteract with surface defects, leading to thermal breakdown. Thedecomposition of the material may lead to pitting or carbon formation,which in turn may lead to increased material damage and rapid failure ofthe optic.

To mitigate LID, optical materials that are relatively tolerant to laserdamage may be selected for use in laser technologies. After the opticalmaterials are selected, screening tests that are specific to aparticular system are conducted. Typically, the tests involve exposingan optic to laser radiation until damage occurs, and then repeating thetests multiple times to collect statistically meaningful informationabout the Laser Induced Damage Threshold (LIDT) of the optic. The LIDTof the optic is defined to be the number of pulses required to damagethe optic, and is measured for a particular coating and optical materialagainst a laser source that is representative of the hardware design(e.g., having the same fluence, wavelength, and pulse width). In someconditions, laser optic materials routinely survive a billion pulses ormore.

One known cause of LID to an optical component is molecular andparticulate contamination. This contamination may result, for example,from outgassing products that condense onto the optical component.Outgassing is the slow release of a gas that was frozen, trapped,absorbed, or adsorbed in some material. Common sources of gas includemoisture, sealants, lubricants, and adhesives, but even metals andglasses can release gases from cracks or impurities. Contaminants,including outgassing products, may degrade optics by causing lighttransmission loss, increased light scatter, and/or obscuration. Whilethe body of research on these contamination effects extends overmultiple decades, a relatively new phenomenon of Contamination EnhancedLaser Induced Damage (C-LID) has only recently gained attention.

C-LID is generally observed when a laser and associated opticalcomponents are enclosed in either a vacuum or sealed gas environment(typically nitrogen or air). As such, C-LID is of particular concernduring the development of space-based optical systems. Such opticalsystems are often developed first as a benchtop version that is afunctional representation of the space-based version. The benchtopversion, however, is not fully analogous to the space-based version,because the benchtop version is typically not enclosed in either avacuum or sealed gas environment. Therefore, it is common for thebenchtop version not to exhibit laser induced damage, and it istypically not until the space-based version is built and enclosed,presumably with the same optical design, that C-LID becomes an issue.For example, it is believed that an open-environment benchtop versionmay not exhibit C-LID because the contaminants, particularly molecularspecies, cannot build up in significant concentrations due to opencirculation throughout the optical cavity of the benchtop system. Incontrast, it is believed that the space-based version may exhibit C-LIDbecause of its vacuum or sealed gas environment. The implication is thatstandard practices for designing, building, and operating benchtopsystem to prevent C-LID may not be fully applicable to space-basedoptical systems when operated in their flight enclosure, e.g., in avacuum or sealed gas environment.

C-LID may cause laser power to rapidly decay and lead to prematurefailure of optical components. Encountered during the development ofspace-based lasers, such as the ones included on the NationalAeronautics and Space Administration (NASA) Mars Orbiter Laser Altimeter(MOLA) and Geoscience Laser Altimeter System (GLAS) missions, C-LID hasalso been observed in laboratory studies. In these reports, opticsexpected to survive well over 1 million pulses from an infrared laserwere observed to fail in as few as 8,000 pulses when contamination wasobserved to be present.

Certain types of contaminants have been observed to cause C-LID,resulting in accelerated damage to optical components. The most commoncontaminants include hydrocarbons and silicones. The most widely studiedcontaminant for C-LID is toluene, also known as methylbenzene. Toluene,a common outgassing compound of epoxies, is relatively volatile, and isa common chemical that has a similar chemical structure to a number ofother aromatic hydrocarbon contaminants. Toluene has been observed toinduce damage on optics, while some other contaminants such as acetone,a common optics cleaning solvent, have not been observed to inducesimilar damage.

Several previously-known systems attempt to address C-LID. U.S. Pat. No.5,770,473 to Hall et al. discloses a package for a high powersemiconductor laser that includes a hermetically sealed container filledwith a dry gaseous medium containing oxygen, for example air having lessthan 5000 ppm water. Hall discloses that the oxygen within the packagingatmosphere serves the important function of minimizing laser damage byorganic impurities. Hall discloses that there is a downside to usingoxygen, namely, that it can react with hydrogen to form water within thelaser enclosure. Additionally, Hall discloses that the water, in turn,can adversely affect the overall operation of the electronic componentswithin the enclosure, including the semiconductor laser, by, forexample, creating a short circuit between the conductors whichinterconnect the components. Hall discloses that the use of a gettermaterial that adsorbs or absorbs water in addition to organicimpurities, such as porous silica and various zeolites, can help tominimize this problem.

Schröder et al., Investigation of UV Laser Induced Depositions on OpticsUnder Space Conditions in Presence of Outgassing Materials, 6th Int'lConf. on Space Optics, held 27-30 Jun. 2006 at ESTEC, Noordwijk (2006)discloses that the outgassing of organic material under vacuumconditions combined with high laser fluences can lead to formation ofdeposits on the optics. Specifically, Schröder discloses aninvestigation of UV-laser induced deposits on uncoated fused silicaoptics in a test chamber under simulated space conditions in thepresence of outgassing materials. Schröder discloses the use of a Nd:YAGlaser and epoxy, silicone, and polyurethane contaminants in theinvestigation. Additionally, Schröder discloses that for testing theinfluence of water on the formation of deposits a liquid reservoir withabout 50 ml was connected via a needle valve to the chamber, and thatthe partial pressure of the water vapor in the chamber was measured witha gas type independent capacitance sensor. Schröder discloses a testwith an epoxy-based contaminant at a partial pressure of 5 mbar waterand compared it with a test without water and stated that water reduceddeposit formation significantly.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide gas phase additives toprevent or reduce Contamination Enhanced Laser Induced Damage (C-LID) tooptical components. Specifically, the gas phase additive may have agreater affinity for the optical component than does the contaminant,and may be present in an amount (e.g. a partial pressure) selected suchthat the additive inhibits binding of the contaminant to the surface ofthe optical component. For example, the additive may be present in anamount selected to substantially cover the surface of the opticalcomponent, leaving substantially no area for the contaminant to adsorbor absorb to the surface. The additive thus may reduce or eliminatedegradation of the optic through mechanisms such as light transmissionloss, increased light scatter, and obscuration. The inhibiteddegradation may result in longer life for the optical component.

In accordance with one aspect of the invention, a device for reducingcontamination laser induced damage to an optical component in a housingcaused by a contaminant includes a container that may be configured tohold a gas phase additive and may be operatively coupled to the housing;and a delivery system that may be configured to introduce the gas phaseadditive from the container into the housing and to maintain the gasphase additive at a pre-selected partial pressure within the housing.The gas phase additive may have a greater affinity for the opticalcomponent than does the contaminant and may be present in an amountsufficient to inhibit laser induced damage resulting from contactbetween the contaminant and the optical component. The housing may beconfigured to maintain a sealed gas environment or vacuum.

Some embodiments further include a sensor that may be configured tosense environmental characteristics within the housing.

In some embodiments, the device includes a controller that may beconfigured to control the delivery system.

In some embodiments, the gas phase additive includes water. In otherembodiments, the gas phase additive includes an alcohol. The alcohol maybe methanol or ethanol.

In some embodiments, the optical component includes a coating. Thecoating may enhance the affinity of the gas phase additive for theoptical component.

In accordance with one aspect of the invention, a method for reducingcontamination laser induced damage to an optical component in a housingcaused by a contaminant includes establishing a sealed gas environmentor vacuum within the housing; providing a container holding a gas phaseadditive; and introducing the gas phase additive from the container intothe housing. The gas phase additive may have a greater affinity for theoptical component than does a contaminant and the gas phase additive maybe present in an amount sufficient to inhibit laser induced damageresulting from contact between the contaminant and the opticalcomponent.

Some embodiments further include sensing pre-determined environmentalcharacteristics of the housing.

In some embodiments, the method includes controlling the introduction ofthe gas phase additives into the housing.

In some embodiments, providing a container may include providing acontainer holding water, alcohol, methanol, and/or ethanol.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A schematically illustrates an exemplary device for reducingContamination Enhanced Laser Induced Damage (C-LID) to an opticalcomponent, according to some embodiments of the present invention.

FIGS. 1B and 1C schematically illustrate exemplary variations of thedevice from FIG. 1A for reducing C-LID to an optical component,according to some embodiments of the present invention.

FIG. 2 illustrates an overview of an exemplary method for reducing C-LIDto an optical component, according to some embodiments of the presentinvention.

FIG. 3 schematically illustrates a high-level view of a C-LID testapparatus.

FIG. 4 schematically illustrates a plan view of a C-LID test chamber foruse in the apparatus of FIG. 3.

FIG. 5 illustrates the results of a Laser Induced Damage Threshold(LIDT) measurement performed on a bare fused silica substrate undervarying laser fluence without any introduced contaminant in the testchamber.

FIG. 6A is a plot illustrating the normalized transmitted energy for aseries of threshold measurements performed on a bare fused silicasubstrate in a flowing mixture of toluene in synthetic air.

FIG. 6B is an image of a damage site produced on the bare fused silicasubstrate from one threshold measurement described in FIG. 6A.

FIG. 7 is a plot illustrating the average number of laser shots measuredto cause a 10% drop in transmitted laser energy performed on bare fusedsilica substrates in varying percentages of oxygen in nitrogen, witherror bars shown.

FIG. 8 is a plot illustrating the average number of laser shots measuredto cause a 10% drop in transmitted laser energy performed on bare fusedsilica substrates in varying concentrations of toluene in synthetic air,with error bars shown.

FIG. 9 is a plot illustrating the normalized transmitted energy for aseries of threshold measurements performed on a bare fused silicasubstrate in a flowing mixture of toluene in synthetic air from FIG. 6Aas compared to the normalized transmitted energy for a series ofthreshold measurements performed on a bare fused silica substrate in aflowing mixture of toluene and water vapor in synthetic air.

FIG. 10 is a plot illustrating the normalized transmitted energy for aseries of threshold measurements performed on a bare fused silicasubstrate in a flowing mixture of toluene and methanol in synthetic air.

FIG. 11 is a plot illustrating the normalized transmitted energy for aseries of threshold measurements performed on a bare fused silicasubstrate in a flowing mixture of toluene and ethanol in synthetic air.

FIG. 12 is a plot illustrating the normalized scatter signal of the HeNelaser and the transmitted Nd:YAG laser energy as a function of thenumber of laser shots.

FIG. 13 is an image of three damage sites produced on a substrate fromthree experiments with flowing mixtures of toluene, nitrogen, oxygen,and/or ethanol.

DETAILED DESCRIPTION

Although techniques to mitigate Laser Induced Damage (LID) have beenimplemented in the past, these techniques may not be applicable orsufficient to prevent or reduce Contamination Laser Induced Damage(C-LID) to optical components in a vacuum or sealed gas environment,e.g., optical components in a space-based laser when operated in itsflight enclosure. The present inventors have recognized that introducingcertain gas phase additives into a housing containing an opticalcomponent that may be exposed to a contaminant may prevent or reduceC-LID. Specifically, the gas phase additive may have a greater affinityfor the optical component than does the contaminant, and may be presentin an amount (e.g. a partial pressure) selected such that the additiveinhibits binding of the contaminant to the surface of the opticalcomponent. For example, the additive may be present in an amountselected to substantially cover the surface of the optical component,leaving substantially no area for the contaminant to adsorb or absorb tothe surface. The additive thus may reduce or eliminate degradation ofthe optic through mechanisms such as light transmission loss, increasedlight scatter, and obscuration. The inhibited degradation may result inlonger life for the optical component.

In some embodiments, the optical component may be treated and/or coatedso as to increase the affinity between the surface of the opticalcomponent and the additive. For example, the component may be cleanedusing a Piranha solution, e.g., a 3:1 mixture of sulfuric acid (H₂SO₄)and hydrogen peroxide (H₂O₂), that both removes most organic matter fromthe optical component and hydroxylates the component's surface. Thishydroxylation will add OH groups to the surface of the opticalcomponent, increasing the hydrophilicity of the surface. A hydrophilicgas phase additive may then be introduced into the optical component'senclosure in an amount sufficient to inhibit laser induced damageresulting from contact between the contaminant and the opticalcomponent, e.g., using a device for reducing C-LID such as describedherein. Because the gas phase additive is selected so as to have agreater affinity for the optical component than does the contaminant,the gas phase additive will bond with the additional OH groups on thesurface of the optical component, thereby inhibiting the contaminantmolecules from binding to the surface. The surface of the opticalcomponent alternatively, or additionally, may be coated with anysuitable coating to which the additive has an affinity that is greaterthan an affinity of one or more contaminants in the enclosure.

FIG. 1A schematically illustrates an exemplary device 100 for reducingC-LID to optical component 110, according to some embodiments of thepresent invention. Device 100 includes housing 120 having component 110disposed therein, laser 130, container 140, delivery system 150,controller 160, and sensor 170.

Optical component 110 may be any optic used with laser 130, including anoptic disposed within laser 130. Optical component 110 may be suitablytreated, e.g., cleaned in a solution known in the art to clean optics,e.g., Piranha solution, before or during use in device 100. Suchtreatment may enhance the affinity of the gas-phase additive to thecomponent's surface. Optical component 110 also, or alternatively, maybe coated with any suitable coating. Such coating may reduce reflectionsof laser light from the surface of optical component 110, and/or mayenhance the affinity of the gas-phase additive to the component'ssurface. Non-limiting examples of optical component 110 include a barefused silica substrate, a silica substrate with an anti-reflective (AR)or other coating, and a lens. Optical component 110 may be disposedwithin housing 120. Housing 120 may be configured to maintain a sealedgas environment or vacuum, and may be any structure suitable forcontaining optics, and in some embodiments is configured for spaceflight.

Laser 130 may be any laser suitable for use in housing 120, e.g., aNd:YAG laser. Laser 130 emits a laser beam that travels through opticalcomponent 110. As noted above, optical component 110 may in someembodiments be part of laser 130. In some embodiments, laser 130 isdisposed within housing 120, and in other embodiments (not shown), laser130 is disposed outside housing 120. When laser 130 is disposed outsidehousing 120, housing 120 may have a window, allowing the laser beam fromlaser 130 to travel into housing 120 and through optical component 110.Housing 120 may have an exit window, allowing the laser beam to travelfrom optical component 110 and through the window to exit housing 120.

Gas phase additive container 140 may be configured to hold a gas phaseadditive and may be operatively coupled to housing 120. The gas phaseadditive may be any substance that has a greater affinity for opticalcomponent 110 than does one or more contaminants within housing 120. Thecontaminant may be any material that induces C-LID. The gas phaseadditive may be in gas phase at the operating temperature of system 100,or may be in a condensed form at the operating temperature of system100, e.g., liquid or solid phase, and suitably vaporized into the gasstate. The gas phase additive may be present in an amount sufficient toinhibit laser induced damage resulting from contact between thecontaminant and optical component 110, for example, in an amountsufficient to substantially cover the surface of optical component 110.Non-limiting examples of the gas phase additive include water vapor,alcohol, methanol, and ethanol. Alcohols such as methanol or ethanol mayin some embodiments be preferred for space-based lasers because theyhave low freezing points as compared to water. Container 140 may be anysuitable structure configured to hold the gas phase additive, whether ingas, liquid, or solid form. In some embodiments, container 140 isdisposed within housing 120, and in other embodiments, container 140 isdisposed outside housing 120. In some embodiments, device 100 includesan apparatus for circulating the gas phase additive in housing 120,e.g., a fan.

Device 100 includes delivery system 150 configured to introduce the gasphase additive from container 140 into housing 120 and to maintain thegas phase additive at a pre-selected partial pressure within housing120. Delivery system 150 may be disposed within housing 120 (not shown)or, as illustrated, coupled to housing 120. Container 140 may bedisposed within delivery system 150, as illustrated, or coupled todelivery system 150 (not shown).

Device 100 further includes controller 160 and sensor 170. Controller160 is operatively coupled to delivery system 150, and is configured tocontrol delivery system 150. Specifically, controller 160 may controlthe quantity and the frequency of gas phase additive introduction tohousing 120 from container 140. Controller 160 may communicate withdelivery system 150 via wired or wireless signals. Controller 160 maybe, for example, a dedicated microcircuit, a processor, or a computer.In some embodiments, controller 160 is disposed within housing 120. Inother embodiments, controller 160 is disposed in a remote locationoutside housing 120 such as on-board a space vessel.

Sensor 170 is operatively coupled to controller 160 and may senseenvironmental characteristics within housing 120, e.g., concentration ofgas phase additive, temperature, and/or pressure. Based on thecharacteristics sensed by sensor 170, controller 160 may determine thequantity and frequency with which to introduce the gas phase additive.Sensor 170 may communicate with controller 160 via wired or wirelesssignals. In some embodiments (not shown), sensor 170 is disposed withinhousing 120. In other embodiments, as illustrated, sensor 170 isdisposed outside housing 120 and operatively coupled to housing 120.

Delivery system 150 may introduce a gas phase additive into housing 120through various mechanisms. In some embodiments, delivery system 150 mayinclude a valve disposed between container 140 and housing 120, thevalve being controlled by controller 160. When the valve is opened, thegas phase additive enters housing 120. Delivery system 150 and/or sensor170 optionally may be detachable from housing 120.

In some embodiments, container 140 may be a pressurized gas bottle, andwhen controller 160 opens the valve, gas phase additives may flow fromthe high pressure environment to the low pressure environment, forexample, from container 140 to housing 120.

In other embodiments, delivery system 150 may include a pressurized gassource and at least one valve, and container 140 may be a bubbler. Abubbler may include a reservoir for a liquid, e.g., a gas phaseadditive, and a pathway for gas to pass through that liquid so as touptake molecules of that liquid and introduce the molecules into ahousing, e.g., housing 120. One valve may be coupled to the pressurizedgas source and the bubbler, and another valve may be coupled to thebubbler and an inlet to housing 120. Controller 160 may open the firstvalve, releasing the pressurized gas into the bubbler which contains thegas phase additive allowing the gas phase additive to attach to the gas.Then, controller 160 opens the second valve to release the pressurizedgas with the gas phase additive into housing 120 via the inlet.

In some embodiments, delivery system 150 may include a heated tubecoupled to an inlet to housing 120, and a dip tube and nozzle. The diptube and nozzle may be coupled to container 140. In this embodiment,container 140 may be a pressurized aerosol bottle containing acondensed, liquid gas phase additive. Controller 160 activates thenozzle allowing gas phase additives to travel into the heated tube viathe dip tube. The heated tube vaporizes the condensed, liquid gas phaseadditives, and from the heated tube, the additives travel into housing120 via the inlet.

In another embodiment, delivery system 150 for introducing the gas phaseadditive into housing 120 may include a pressurized gas source, at leastone valve, and a heater. In this embodiment, container 140 is disposedbetween the pressurized gas source and an inlet to housing 120.Container 140 holds a high surface area solid called a sorbent, e.g.,Tenax, zeolite, polymer, or a metal oxide framework, and the gas phaseadditive is adsorbed onto the sorbent. The heater heats the sorbent todesorb the gas phase additive into the atmosphere. Controller 160 mayrelease gas from the pressurized gas source which travels through theheated sorbent and into housing 120. When the gas travels through thesorbent, desorbed gas phase additive particles are carried with the gasinto housing 120. A first valve disposed between the pressurized gassource and the sorbent, and a second valve disposed between the sorbentand the inlet to housing 120 may be controlled by controller 160 topulse gas into housing 120. Alternatively, an apparatus for circulatingthe gas phase additive in housing 120, e.g., a fan, may be used in placeof the pressurized gas source.

Delivery system 150 may maintain the gas phase additive at apre-selected partial pressure using controller 160 and sensor 170.Specifically, when sensor 170 measures the pressure within housing 120,delivery system 150 may introduce the gas phase additive into housing120 as directed by controller 160 if the pressure in housing 120 is notthe pre-selected partial pressure. The introduction of the gas phaseadditive will adjust the partial pressure to that of the pre-selectedpartial pressure as directed by controller 160.

FIG. 1B schematically illustrates an exemplary variation of device 100from FIG. 1A for reducing C-LID to optical component 110, according tosome embodiments of the present invention. Device 101 may be designed ina similar manner as device 100 from FIG. 1A, except device 101 mayfurther include two sensors 170, optical housing 121, and laser housing122. Optical component 110 is disposed within optical housing 121, whilelaser 130 (which may include its own optics subject to C-LIDdegradation) is disposed within laser housing 122. Because an additionalhousing (laser housing 122) is included in device 101, an additionalsensor 170 may be included to sense characteristics within that housing,e.g., concentration of gas phase additive, temperature, and/or pressure.Delivery system 150 may further introduce the gas phase additive intoboth laser housing 122 and optical housing 121 and maintain the gasphase additive within the respective housings at a pre-selected partialpressure. These variations are illustrated in FIG. 1B where opticalhousing 121 is coupled to sensor 170 and delivery system 150, and laserhousing 122 is coupled to sensor 170 and delivery system 150. Anysuitable number of housings and sensors may be provided, with deliverysystem 150 coupled thereto.

FIG. 1C schematically illustrates another exemplary variation of device100 from FIG. 1A for reducing C-LID to optical component 110, accordingto some embodiments of the present invention. Device 102 may be designedin a similar manner as device 100 from FIG. 1A, except container 140,delivery system 150, controller 160, and sensor 170 are illustrated asdisposed within the housing, referred to as housing 123. In thisembodiment, delivery system 150 introduces a gas phase additive fromcontainer 140 into housing 123. As described above with respect to FIG.1A, each of laser 130, controller 160, and/or sensor 170 may be disposedeither inside or outside of housing 123, according to variousembodiments of the present invention.

Devices 100, 101, and 102 may be used, for example, in a wide range ofapplications. These applications may include, but are not limited to,laser ranging, laser altimetry, Light Detection and Ranging (LIDAR),laser communication, laser sensing, and/or laser power beaming. Suchapplications may be space-based, may be based on a mobile platform suchas an aircraft or ground-based vehicle, or may be associated with afixed location. Other applications may include medical lasers forsurgery and other procedures, and high power lasers for laser fusion.

FIG. 2 illustrates an exemplary method 200 for reducing C-LID to anoptical component in an enclosed housing, according to some embodimentsof the present invention. First, at step 210, a housing establishes asealed gas environment or vacuum about the optical component. Thehousing may be sealed using suitable techniques known in the art tocreate a sealed gas environment. For example, the housing may be in avacuum because the housing is in space. Or, for example, a vacuum may beachieved within the housing suitable techniques known in the art, e.g.,using a turbo-pump or rough pump.

Next, at step 220, a container configured to hold a gas phase additiveis provided. The container may be coupled to the housing and may bedisposed within the housing. As noted above with respect to FIG. 1A, thecontainer may be any structure configured to hold the gas phaseadditive, whether in gas, liquid, or solid form. Non-limiting examplesof the container include a pressurized bottle, a bubbler, an aerosolbottle, or a device for holding a sorbent. The gas phase additive may beany substance that has a greater affinity for the optical component thandoes one or more contaminants within the housing. The contaminant may beany material that induces C-LID. The gas phase additive may be in gasphase form or in a condensed form, e.g., liquid or solid state. The gasphase additive may be present in an amount sufficient to inhibit laserinduced damage resulting from contact between the contaminant and theoptical component. Non-limiting examples of the gas phase additiveinclude water, alcohol, methanol, and ethanol. Alcohols such as methanolor ethanol are useful for space-based lasers because they have lowfreezing points as compared to water.

Finally, at step 230, the gas phase additive from the container isintroduced into the housing. As noted with respect to FIG. 1A, thisintroduction may be achieved by using at least one delivery system,controller, and sensor, according to some embodiments of the presentinvention. The container provided may be suitably connected to thedelivery system, allowing the gas phase additive to be introduced to thehousing via the delivery system. The controller controls the deliverysystem, and specifically controls the quantity and the frequency of thegas phase additive introduction to the housing from the container. Thesensor senses pre-determined environmental characteristics within thehousing, e.g., concentration of gas phase additive, temperature, and/orpressure. As described above, the introduction of gas phase additivesinto the housing may include releasing the gas phase additive into thehousing via an optionally detachable delivery system; releasing thepressurized gas phase additive into the housing; flowing a gas through abubbler such that the gas includes the gas phase additive and releasingthe gas into the housing; releasing a gas phase additive into thehousing an aerosol device; or flowing a gas through a sorbent with thegas phase additive adsorbed onto the sorbent and into the housing.

In some embodiments, method 200 may further include generating a laserbeam with a laser, e.g., a Nd:YAG laser. The laser beam travels throughthe optical component. Method 200 reduces or prevents C-LID to theoptical component from the laser beam.

In some embodiments, providing a container at step 220 may includeproviding a container holding water, alcohol, methanol, and/or ethanol.

Method 200 may be used, for example, in a wide range of enclosed-housinglaser applications, including space-based laser applications. Theseapplications may include, but are not limited to, laser ranging, laseraltimetry, Light Detection and Ranging (LIDAR), laser communication,laser sensing, and/or laser power beaming. Other applications mayinclude medical lasers for surgery and other procedures, and high powerlasers for laser fusion.

EXAMPLE

The functionality of devices 100, 101, and 102 and method 200 forreducing C-LID to an optical component in a housing may be illustratedby way of Example as described below.

1. Experimental Set-Up

FIG. 3 is a schematic view of C-LID test apparatus 300, which may beused to measure Laser Induced Damage Threshold (LIDT) and the extent ofC-LID on bare fused silica substrates in vacuum conditions. Apparatus300 includes Helium Neon (HeNe) laser 310, which generates HeNe laserbeam 311; mirrors 320, 321, 322; Nd:YAG laser 330, which generatesNd:YAG laser beam 331; dichroic beam splitter 340; energy meters 350,395; irises 360, 361; high-power laser mirrors 370, 371, 390; telescope380; and test chamber 400 which, as described in greater detail below,contains the bare fused silica substrate.

The laser beams in apparatus 300 were aligned before entering testchamber 400. HeNe laser 310 emits HeNe laser beam 311. Mirrors 320, 321,322 reflect HeNe laser beam 311 to an appropriate point on dichroic beamsplitter 340. Nd:YAG laser 330 emits Nd:YAG laser beam 331. Nd:YAG laser330 is a flash-lamp pumped, 1064 nm pulsed Nd:YAG laser (e.g., ContinuumPowerlite II), having a repetition rate of 20 Hz, and a nominal diameterof 10 mm, as determined from burn paper. Nd:YAG laser beam 331 travelsto dichroic beam splitter 340 where dichroic beam splitter 340 splitsNd:YAG laser beam 331 into two parts. One part of Nd:YAG laser beam 331travels to energy meter 350, which measures the transmitted laser energyfrom Nd:YAG laser beam 331. The second part of Nd:YAG laser beam 331meets with HeNe laser beam 311 at dichroic beam splitter 340 so that thetwo beams 311, 331 travel collinearly through the remainder of thesystem, allowing HeNe laser beam 311 to be used to align Nd:YAG laserbeam 331 (illustrated as combined beam 341). Beam 341 travels to irises360, 361 which are used to align beam 341. High-power laser mirrors 370,371 appropriately direct beam 341 through telescope 380 (e.g., aGalilean telescope). Telescope 380 is used to collimate beam 341,resulting in a reduction in beam diameter that decreases laser powerloss from light scattering. High-power laser mirror 390 directs beam 341through test chamber 400.

When beam 341 is properly aligned, it enters test chamber 400 and passesthrough the bare fused silica substrate. Energy meter 395 measures thelaser energy transmitted through chamber 400 and the bare fused silicasubstrate.

FIG. 4 schematically illustrates a plan view of an exemplary C-LID testchamber 400 suitable for use with apparatus 300 illustrated in FIG. 3.Chamber 400 is used to expose substrate 402 (e.g., a bare fused silicasubstrate) to a laser beam in vacuum conditions and to introduce gasphase additives. Chamber 400 includes exposure chamber 401 havingsubstrate 402 disposed therein, gas source 411, exhaust system 412,turbomolecular pump 415, roughing pump 416, contaminant bubbler 430,additive bubbler 432, and various view ports, valves, and pieces ofmeasuring equipment, some of which are described in greater detail belowand the remainder of which will be familiar to those of ordinary skillin the art of vacuum chambers.

The substrate 402 to be tested, e.g., a bare fused silica substrate, waspositioned within exposure chamber 401 so as to allow the substrate tobe exposed to the laser beam 341. Exposure chamber 401 is stainlesssteel and consists of a 6″ cross. Substrate 402 is a 2″ diameter fusedbare fused silica window that was cleaned in a Piranha solutionconsisting of sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂).Exposure chamber 401 contains a fixture (not shown) for holding andtranslating substrate 402 so as to allow automated positioning andtesting. View ports 403, 404, 405, 406, 407, 408, e.g., silica windows,are coupled to the exposure chamber 401 via stainless steel tubes withadapter flanges, and are used to visually align substrate 402 withinexposure chamber 401. Entrance view port 407 allows beam 341 to travelinto test chamber 400 and through substrate 402, while exit view port408 allows beam 341 to exit test chamber 400 and travel to energy meter395 following transmission through substrate 402.

To reduce the risk of damage to entrance view port 407 and/or exit viewport 408 by beam 341 during the experiment, purge valves 409, 410 arecoupled to entrance view port 407 and exit view port 408, respectively.Purge valves 409, 410 may be opened to purge their respective view port407, 408 with atmospheric gas from gas source 411 via gas lines (notshown), to inhibit contamination buildup. As a result, the transmittedlaser energy of beam 341 was believed to be unaffected by damage toentrance view port 407 or exit view port 408.

Test chamber 400 includes exhaust system 412 coupled to exhaust valve413 via vacuum line 414. Exhaust system 412 is maintained nearatmospheric pressure, allowing any introduced gas to continuously flowthrough the chamber and exit via exhaust system 412. Exhaust valve 413can be closed to achieve vacuum conditions within test chamber 400.

Vacuum conditions are achieved using turbomolecular pump 415 androughing pump 416 to simulate a space-based laser operated in its flightenclosure, thus allowing C-LID to be observed. Turbomolecular pump 415is coupled to exposure chamber 401 via a stainless steel tube with anadapter flange, while roughing pump 416 is coupled to roughing valve 417via vacuum line 418. Gate valve 419 is used to isolate turbomolecularpump 415 from test chamber 400 while the lasers are activated during theexperiments. Both pumps remain on for the full duration of anexperiment.

The process for achieving vacuum conditions within test chamber 400begins by closing purge valves 409, 410, closing exhaust valve 413, andshutting off a gas valve (not shown) at gas inlet 420. Roughing valve417 is opened, allowing roughing pump 416 to reduce the pressure withinexposure chamber to approximately 100 mTorr. Roughing valve 417 isclosed and gate valve 419 is opened, allowing turbomolecular pump 415 toreduce the pressure in the chamber to the microTorr region, therebyachieving vacuum conditions. Gate valve 419 is then closed.

Test chamber 400 includes several pieces of measuring equipment used tomonitor the vacuum conditions. Residual gas analyzer (RGA) 421 iscoupled to turbomolecular pump 415 via a stainless steel tube with anadapter flange, and monitors the quality of the vacuum and detectsminute traces of impurities. RGA 421 is coupled to the turbo-pumpedportion of the chamber, and operates between 1×10⁻⁸ Torr and 1×10⁻⁴Torr. Vacuum line 422 is connected between RGA 421 and exhaust valve413, allowing gases to travel to RGA 421 for measurement after goingthrough exposure chamber 401, while gate valve 419 is closed. Ion gauges423, 424 are coupled to turbomolecular pump 415, and measure thepressure within test chamber 400 at their respective locations.

After vacuum conditions are achieved, the chamber is filled with gas tocreate a controlled flowing gas environment. This is accomplished bybackfilling test chamber 400 with a gas, e.g., nitrogen and/or oxygen,from gas source 411 via gas line 425, and opening the gas valve, purgevalves 409, 410, exhaust valve 413, and butterfly valve 426. Gas fromgas source 411 flows into the chamber via the gas valve and purge valves409, 410, travels through the chamber, and exits through exhaust system412. There are several pieces of equipment that can be used to monitorthe flowing gas environment, including capacitance manometers 427, 428,429, which measure pressure. Each manometer 427, 428, 429 isrespectively calibrated to accurately measure a pressure range, e.g., upto 1 Torr, up to 50 mTorr, or up to 1000 Torr, respectively. In theseexperiments, only manometer 429 was used.

After the chamber is backfilled, contaminants, if any, and gas phaseadditives, if any, can be introduced into the chamber via gas line 425.The contaminants, e.g., toluene, are introduced so as to induce C-LID onsubstrate 402 in a manner that simulates C-LID on optical components inother enclosed laser systems, e.g. space-based systems. The contaminantsare disposed within temperature-controlled contaminant bubbler 430, andmay be introduced into the chamber by opening bubbler valve 431. The gasphase additives, e.g., water vapor, methanol, or ethanol, are introducedto determine what effects, if any, the additives have on C-LID onsubstrate 402. The additives are disposed within temperature-controlledadditive bubbler 432 in condensed (liquid) form and may be introducedinto the chamber by opening bubbler valve 433. Once bubbler valves 431,433 are opened, gas from gas source 411 flows through bubblers 430, 432via gas lines (not shown), causing the contaminant and gas phaseadditive to flow into chamber 400. Approximately one hour was allottedto allow test chamber 400 to reach steady state.

2. Experimental Parameters

For each experiment, test apparatus 300 and test chamber 400 wereutilized. After a high level vacuum was achieved and the proper gasphase additives were introduced, substrate 402 was exposed to beam 341until the transmitted energy of Nd:YAG laser beam 331 as measured byenergy meter 395 fell to 80% of its starting value or until 1×10⁶ laserpulses were reached. If Nd:YAG laser 330 generated 1×10⁶ laser pulses at20 Hz and the transmitted energy did not fall to 80% of its startingvalue, each experiment lasted 13.9 hours. The pulse energy of Nd:YAGlaser 330 was adjusted using a combination of partial reflectors andadjusting the Q-switch timing. The pulse width of Nd:YAG laser 330 wasmeasured to be 13 ns from a digitizing oscilloscope (not shown) and afast photodiode (not shown). Each experiment was repeated at least ninetimes for each substrate 402 and each set of conditions. Substrate 402was automatically translated vertically in approximately 5 mm steps viathe fixture and beam 341 was translated approximately 1 cm to create twocolumns of at least nine exposure spots across substrate 402. Aftersubstrate 402 received nine to ten exposure spots, a different,virtually identical substrate 402 was utilized for experimentation.

3. Laser Induced Damage Threshold without Contaminant

FIG. 5 illustrates the results of a LIDT measurement performed onsubstrate 402 under varying laser fluence without any introducedcontaminant in test chamber 400. In this experiment, beam 341 wasfocused behind substrate 402 to give a beam diameter at substrate 402 ofabout 2 mm. FIG. 5 shows a plot of the laser energy in Joules of Nd:YAGlaser 330 as measured by energy meter 395 versus the number of laserpulses, and an image of the laser ablation. As seen in FIG. 5, substrate402 was stable until the laser fluence reached approximately 8.4 J/cm².At this value, the power became high enough to cause laser ablation,cracking and breaking the surface of substrate 402 within a few seconds.The inset of FIG. 5 is an image of this laser ablation, showing a holethat went completely through substrate 402 that was the size of the beamdiameter. The image further shows that at the edge of substrate 402,there was cracking and breaking of substrate 402.

4. Laser Induced Damage Threshold with Contamination: Toluene

The next experiments determined the LIDT of substrate 402 with acontaminant in the test chamber 400. The chosen contaminant was toluene.The selected transmitted energy of the Nd:YAG laser 330 was 85 mJ,making the fluence approximately 4 J/cm². This fluence was chosen as itwas lower than the measured LIDT of the substrate, as discussed abovewith respect to FIG. 5, and is typical of fluences used in space flightlaser systems.

Each experiment included preparing test apparatus 300 and test chamber400 as described above with respect to vacuum achievement, andintroducing toluene from contaminant bubbler 430 mixed with nitrogen(N₂) and oxygen (O₂), if any, from gas source 411 and a gas phaseadditive, if any, from additive bubbler 431 by way of gas line 425 viagas inlet 420. The gas phase additives selected were water vapor,methanol, and ethanol. A number of experiments were performed: toluenein nitrogen, toluene in synthetic air (20% O₂ and 80% N₂), toluene insynthetic air with water, toluene in synthetic air with methanol, andtoluene in synthetic air with ethanol.

After the experiments were performed, plots were created showing thenormalized transmitted energy versus the number of laser pulses (soreferred to as “shots”) to damage for each experiment. The damagethreshold was then calculated for each experiment. The calculated damagethreshold was taken as the average number of laser shots, includingerror, required to cause a 10% drop in transmitted laser energy. Anoptical component that experienced a 10% drop in transmitted laserenergy would be considered a failure to one of ordinary skill in theart.

FIG. 6A is a plot illustrating the normalized transmitted energy for aseries of threshold measurements performed on substrate 402 in a flowingmixture of toluene in synthetic air. The concentration of toluene in theflowing mixture was 300 parts per million (ppm). The result of thisexperiment was a calculated damage threshold for substrate 402 in theflowing mixture of toluene in synthetic air of (1.8±0.2)×10⁴ shots.

FIG. 6B is an image of a damage site produced on substrate 402 from onethreshold measurement described in FIG. 6A. As seen in FIG. 6B, toluenedamaged substrate 402 creating a black ring structure that was notdamaged in the center. The black ring was graphitic carbon as determinedby Raman spectroscopy and, without wishing to be bound by any theory,the inventors attribute the black ring to radical formation from thetoluene as a result of laser excitation.

FIG. 7 is a plot illustrating the average number of laser shots measuredto cause a 10% drop in transmitted laser energy performed on bare fusedsilica substrates in varying percentages of oxygen in nitrogen, witherror bars shown. The concentration of toluene in the flowing mixtureswas 300 ppm. The calculated damage thresholds for varying percentages ofoxygen in nitrogen are summarized in TABLE 1.

TABLE 1 Oxygen Nitrogen Calculated Damage Number of PercentagePercentage Threshold (shots) Experiments 0 100 (7.8 ± 1.8) × 10³ 15 2080 (1.8 ± 0.2) × 10⁴ 9 33 67 (2.4 ± 0.3) × 10⁴ 9 67 33 (3.7 ± 0.9) × 10⁴10

As seen in FIG. 7 and TABLE 1, the calculated damage threshold forsubstrate 402 increased as the percentage of oxygen in exposure chamber401 increased. Without wishing to be bound by any theory, the inventorsattribute this to the quenching and/or reactivity of oxygen withradicals that may be formed preventing soot formation.

FIG. 8 is a plot illustrating the average number of laser shots measuredto cause a 10% drop in transmitted laser energy performed on bare fusedsilica substrates in varying concentrations of toluene in synthetic air,including error bars. The calculated damage thresholds for varyingconcentrations of toluene in synthetic air are summarized in TABLE 2.

TABLE 2 Toluene Calculated Damage Number of Concentration (ppm)Threshold (shots) Experiments 50 (1.3 ± 1.2) × 10⁵ 9 100 (3.8 ± 1.0) ×10⁴ 10 300 (1.8 ± 0.2) × 10⁴ 9

As seen in FIG. 8 and TABLE 2, the calculated damage threshold forsubstrate 402 decreased as the toluene concentration in exposure chamber401 increased. FIG. 8 verifies that toluene induces damage on optics.There was a large error in the calculated damage threshold at 50 ppmwhich, without wishing to be bound by any theory, the inventorsattribute to inadequate temperature control of contaminant bubbler 430.

5. Laser Induced Damage Threshold with Contaminant and Gas-PhaseAdditive

FIG. 9 is a plot illustrating the normalized transmitted energy 910 fora series of threshold measurements performed on a bare fused silicasubstrate in a flowing mixture of toluene in synthetic air from FIG. 6A.This result is compared to the normalized transmitted energy 920 for aseries of threshold measurements performed on a bare fused silicasubstrate in a wet flowing mixture of toluene and water vapor insynthetic air. In this wet flowing mixture, the concentration of toluenewas 300 ppm and the concentration of water vapor was 3400 ppm.

In this wet flowing mixture experiment, substrate 402 was observed tolast for greater than 1×10⁶ shots in nine out of ten experiments.Without wishing to be bound by any theory, the inventors attribute theexperiment where the normalized transmitted energy fell below 90% toparticulate contamination. This failure was at 3.9×10⁵ shots, giving acalculated minimum damage threshold of greater than (9.4±1.9)×10⁵ shots.The calculated damage threshold is considered a minimum thresholdbecause most of the experiments were stopped before failure wasobserved. Even so, the calculated minimum damage threshold is a dramaticincrease in the life of substrate 402 as compared to the calculateddamage threshold of (1.8±0.2)×10⁴ shots for the dry flowing mixture.

FIG. 10 is a plot illustrating the normalized transmitted energy for aseries of threshold measurements performed on a bare fused silicasubstrate in a wet flowing mixture of toluene and methanol in syntheticair. In this wet flowing mixture, the concentration of toluene was 300ppm and the concentration of methanol was 4500 ppm. As seen in FIG. 10,the normalized transmitted energy 1010 did not fall below 90% before1×10⁶ shots in nine out of ten experiments; however the substrate failedat approximately 3.8×10⁵ shots in one experiment (1020). When the wetflowing mixture with methanol was introduced, the calculated minimumdamage threshold was greater than (9.3±2.2)×10⁵ shots. This calculatedminimum damage threshold is also a dramatic increase in the life ofsubstrate 402 as compared to the calculated damage threshold of(1.8±0.2)×10⁴ shots for the dry flowing mixture.

FIG. 11 is a plot illustrating the normalized transmitted energy 1110for a series of threshold measurements performed on a bare fused silicasubstrate in a wet flowing mixture of toluene and ethanol in syntheticair. In the wet flowing mixture, the concentration of toluene was 300ppm and the concentration of ethanol was 4900 ppm. As seen in FIG. 11,the normalized transmitted energy 1110 did not fall below 90% before1×10⁶ shots in all eleven runs. When the wet flowing mixture withethanol was introduced, the calculated minimum damage threshold wasgreater than 1×10⁶ shots. This calculated minimum damage threshold is aneven more dramatic increase in the life of substrate 402 as compared tothe calculated damage threshold of (1.8±0.2)×10⁴ shots for the dryflowing mixture.

The calculated damage thresholds for different tested flowing mixturesin synthetic air with a 300 ppm concentration of toluene are summarizedin TABLE 3.

TABLE 3 Gas Phase Additive Calculated Damage Number of (concentration inppm) Threshold (shots) Experiments None 18,000 ± 2,100 9 Water Vapor(3400) >940,000 ± 190,000 10 Methanol (4500) >930,000 ± 220,000 10Ethanol (4900) >1,000,000 11

As seen in TABLE 3, the calculated damage threshold increasesdramatically when certain gas phase additives are introduced to testchamber 400. Without wishing to be bound by any theory, the inventorsattribute this increase to the introduction of a gas phase additive thathas a greater affinity for an optical component, e.g., substrate 402,than does a contaminant, e.g., toluene. The additive inhibits binding ofthe contaminant to the surface of the optical component, thus reducingor eliminating degradation of the optic through mechanisms such as lighttransmission loss, increased light scatter, and obscuration. Theinhibited degradation results in longer life for the optical component.

FIG. 12 is a plot illustrating the transmitted Nd:YAG laser energy 1210as a function of the number of laser shots and the normalized scattersignal 1220 of HeNe laser 310. Laser energy 1210 was calculated as thelaser energy measured by energy meter 395 over the laser energy measuredby energy meter 350 from FIG. 3. To comply with InternationalOrganization for Standardization (ISO) 11254: Lasers and Laser-RelatedEquipment (2000), laser energy 1210 and scatter signal 1220 werecalibrated such that their normalized transmitted laser energy wouldintersect at approximately 0.90, which occurred at 37,358 laser shots.It should be noted that after approximately 40,000 laser shots, scattersignal 1220 flattened out which the inventors attribute, without wishingto be bound by any theory, to saturation of the photodiode and/oramplifier.

FIG. 13 is an image of three damage sites 1310, 1320, 1330 produced onsubstrate 402 from three experiments with flowing mixtures of toluene,nitrogen, oxygen, and/or ethanol. Damage site 1310 resulted from anexperiment where the flowing mixture included nitrogen and 300 ppm oftoluene. Damage site 1310 has a dark black ring, an indication of C-LID.Damage site 1320 resulted from an experiment where the flowing mixtureincluded synthetic air, e.g., 80% N₂ and 20% O₂, and 300 ppm of toluene.Damage site 1320 has a black ring exhibiting C-LID less severe thandamage site 1310. Damage site 1330 resulted from an experiment where theflowing mixture included 300 ppm of toluene, synthetic air, e.g., 80% N₂and 20% O₂, and 4900 ppm of ethanol. As seen in FIG. 13, the distancebetween damage site 1310 and damage site 1330 was measured to be 12 mm.Damage site 1330 shows no discoloration and substrate 402 may not bedamaged. The inventors attribute, without wishing to be bound by anytheory, the inhibited damage to the introduction of ethanol into theflowing mixture.

While preferred embodiments of the invention are described herein, itwill be apparent to one skilled in the art that various changes andmodifications may be made. The appended claims are intended to cover allsuch changes and modifications that fall within the true spirit andscope of the invention.

What is claimed:
 1. A system for inhibiting contamination enhanced laserinduced damage caused by a laser beam, the system comprising: a housingconfigured to maintain a sealed gas environment or vacuum, having a gasphase contaminant therein, and having a laser source therein, whereinthe laser source generates the laser beam; an optical component,disposed within the housing, that is a part of the laser source orthrough which the laser beam travels, wherein the optical component istransmissive and controls a path of the laser beam; a water or alcoholcontainer operatively coupled to the housing; a delivery systemconfigured to introduce water or alcohol in a gas phase into the housingfrom the container; a sensor configured to sense an environmentalcharacteristic within the housing and to generate an output based on thesensed environmental characteristic; and a controller, disposed withinthe housing, configured to receive the output of the sensor and based onthe output of the sensor to cause the delivery system to introduce gasphase water or alcohol into the housing from the container in an amountsufficient to generate and maintain a layer of the water or alcoholadsorbed to the optical component from the gas phase and covering asurface of the optical component, the layer leaving minimal area on thesurface of the optical component via which the contaminant adsorbs orabsorbs to the surface and displacing the contaminant from the surfaceof the optical component so as to inhibit damage to the opticalcomponent resulting from contact between the contaminant, the opticalcomponent, and the laser beam.
 2. The system of claim 1, wherein thealcohol comprises methanol.
 3. The system of claim 1, wherein thealcohol comprises ethanol.
 4. The system of claim 1, wherein the opticalcomponent includes a coating, wherein the coating reduces reflections ofthe laser beam.
 5. The system of claim 1, wherein the optical componentincludes a coating, the coating enhances an affinity of the gas phasewater or alcohol to adsorb to the optical component.
 6. The system ofclaim 1, wherein the gas phase contaminant comprises toluene.
 7. Thesystem of claim 1, wherein the optical component having the water oralcohol adsorbed thereto transmits at least 1 million laser shotstherethrough minimizes contamination enhanced laser induced damage(C-LID) to the optical component.
 8. The system of claim 1, wherein thedelivery system introduces the gas phase water or alcohol into thehousing at a concentration of at least 3400 ppm.
 9. The system of claim1, wherein the optical component comprises fused silica.
 10. The systemof claim 1, wherein the water or alcohol a sorbent is in a condensedphase within the container and wherein the delivery system is configuredto volatilize the condensed phase water or alcohol into the gas phase.11. A method for reducing contamination enhanced laser induced damagecaused by a laser beam, the method comprising: providing a housingmaintaining a sealed gas environment or vacuum having a gas phasecontaminant therein, and having a laser source therein, wherein thelaser source generates the laser beam; disposing within the housing anoptical component that is transmissive and is a part of the laser sourceor through which the laser beam travels; operatively coupling to thehousing a water or alcohol container; providing a delivery system;introducing water or alcohol in a gas phase via the delivery system intothe housing from the container; providing a sensor; sensing anenvironmental characteristic within the housing via the sensor andgenerating an output based on the sensed environmental characteristic;providing a controller, disposed within the housing; and receiving theoutput of the sensor with the controller and based on the output of thesensor controlling the delivery system to introduce gas phase water oralcohol into the housing from the container to generate and maintain alayer of the water or alcohol adsorbed to the optical component thatcovers a surface of the optical component, the layer leaving minimalarea on the surface of the optical component via which the contaminantadsorbs or absorbs to the surface and displacing the contaminant fromthe surface of the optical component so as to inhibit damage to theoptical component resulting from contact between the contaminant, theoptical component, and the laser beam.
 12. The method of claim 11,wherein the alcohol comprises methanol.
 13. The method of claim 11,wherein the alcohol comprises ethanol.
 14. The method of claim 11,wherein the optical component includes a coating, wherein the coatingreduces reflections of the laser beam.
 15. The method of claim 11,wherein the optical component includes a coating, the coating enhancesan affinity of the gas phase water or alcohol to adsorb to the opticalcomponent.
 16. The method of claim 11, wherein the gas phase contaminantcomprises toluene.
 17. The method of claim 11, comprising transmittingat least 1 million laser shots through the optical component having thewater or alcohol adsorbed thereto minimizes contamination enhanced laserinduced damage (C-LID) to the optical component.
 18. The method of claim11, wherein the gas phase water or alcohol is introduced to the housingat a concentration of at least 3400 ppm.
 19. The method of claim 11,wherein the optical component comprises fused silica.
 20. The method ofclaim 11, wherein the water or alcohol is in a condensed phase withinthe container and wherein the introducing step comprises volatilizingthe water or alcohol into the gas phase.